The control of the resonant interaction between a free shear layer and a cavity is of direct relevance to many wind-tunnel testing and aircraft applications. Acoustic levels in zones of local flow separation such as gaps, cavities, and junctures can generate pure tone acoustic components having large amplitudes which, at a minimum, contaminate acoustic field measurements and which, in the extreme, lead to fatigue of components and systems. Such cavities exist in landing gear bays, weapon delivery systems, optics bays, at junctions between wind tunnel model components, and in a variety of instrument installation configurations. Cavity noise is a major reliability and maintainability issue in several aircraft programs, and can be a dominant factor in determining the success of programs with instrumentation in cavities. Thus, the control of internal cavity dynamic loads is an issue of critical importance.
The complex nature of cavity flows begins when a thick (usually turbulent) boundary layer separates at the upstream or leading edge of a cavity formed in a portion of a structure. Local conditions such as the shape of the leading edge control the actual separation location. A leading edge that is sharp fixes the separation location but also enhances shear-layer receptivity to acoustic disturbances. The unsteady characteristics of the resulting free shear layer over the cavity are determined by the mean profile and turbulence characteristics of the incoming boundary layer as well as the disturbances imposed on the shear layer through the receptivity process. The shear layer over the cavity develops based on the separating shear layer conditions and the instability characteristics of the mean shear layer profile. Velocity profile shaping can be used with some success to move the amplification band away from those frequencies tuned to cavity resonance.
The leading edge of the cavity is the significant location for acoustic receptivity which is defined as the process by which long-wavelength acoustic disturbances couple with the shorter-wavelength disturbances in the separating free shear layer. When a leading edge is sharp, the shear layer is highly susceptible to the unsteady pressure gradients imposed by the interaction between the incident acoustic field and the leading edge. If the leading edge is blunt, it produces significantly lower receptivity to externally imposed acoustic fields.
The shear layer subsequently reattaches to the surface of the structure at the aft end or trailing edge of the cavity which serves as the primary acoustic source. In cases where reattachment is delayed until past the trailing edge, the reattachment is more benign and the acoustic levels are reduced in amplitude. Rounded or perforated trailing edges have been used to modify the reattachment zone and decrease the amplitude of the acoustic disturbance field.
The sound produced when the shear layer reattaches to the aft cavity wall provides the primary acoustic source that drives the cavity acoustics. The geometric shape of the cavity determines which specific acoustic modes dominate. For example, a cavity having regular internal dimensions will produce the greatest resonant amplitudes. Since the shear layer provides a wide range of source frequencies, there exists the possibility that natural cavity resonances will be stimulated. Irregular cavity dimensions will reduce the peak acoustic amplitudes, but in turn will ensure that resonance conditions exist over a wide range of frequencies and operating conditions. Thus, passive geometric modifications to the cavity or its surrounding environment will not necessarily lead to a solution of the resonance problem over a wide range of frequencies and operating conditions.
Another source of resonance in the cavity is the feedback of energy to the leading edge where the initial separation occurs. The amplitude and frequency content of the feedback ultimately controls the shear layer disturbance. Some reduction of feedback can be achieved through the use of sound-absorbing cavity liners. However, while reducing resonance amplitude and generation of tones, such liners are typically not effective at low acoustic frequencies because the thickness of the liners becomes large compared to the cavity dimensions. Therefore, additional noise reduction mechanisms are generally used in conjunction with such liners.
Acoustic amplitude reduction can also be achieved by introducing cancellation noise from one or more acoustic sources. This approach has successfully been employed in the prior art for reduction of low-frequency components of noise emitted from exhaust systems, for ambient noise reduction in headsets, and for localized noise reduction in aircraft interiors. When the acoustic sources are configured with the appropriate phase, amplitude, and frequency content, acoustic levels can be minimized within certain constraints. For example, in duct propagation where plane waves are the dominant component, the plane waves can effectively be canceled with a limited number of sources. However, in more complex three-dimensional environments such as cavities, it is only possible to minimize the noise at a limited number of locations. In essence, the number of active sources controls the number of degrees of freedom available for active cancellation. Thus, active cancellation is not a practical option for lowering acoustic levels within an entire cavity. In addition, the large sound levels encountered in the cavity require impractical power inputs for effective sound cancellation.